1. Field of the Invention
This invention relates to a method of detecting circuit discontinuities in conductor members supported by glass sheets, such as in a defrosting automobile back light.
2. Description of the Technical Difficulties
Windows for automobiles, and particularly back lights, used for automobiles are commonly provided with electric heating elements for defrosting and defogging. Such back lights are generally made from a pattern cut flat glass sheet thermally bent or molded to fit the particular automobile window opening. Prior to the heating and bending of the glass sheet, a number of narrow spaced parallel lines of a conducting material is placed on the inside vision area of the glass. The parallel lines are connected on opposite adjacent margins thereof by a strip of electrodes of the same material which is generally a ceramic frit containing silver or silver oxide. Upon the heat of bending and tempering, the lines and strip electrode buses are fired onto the glass so that the pattern becomes part of the glass substrate. When the automobile electrical power source is connected to the strip electrode buses, the resultant resistant heating in the parallel strips defogs or defrosts the glass.
Normally the silver/silver oxide frit is applied by silk screening the pattern onto one surface of the glass sheet while it is still flat. During the subsequent treatment of the sheet by heating and bending the glass to the desired curvature, the frit material becomes tightly bonded to the glass surface.
Other glass sheets or substrates, such as aircraft transparencies have embedded in them a plurality of very fine conductor members, these being of copper or the like. The conductor members are of size, number, spacing and shape so as to not interfere with the transmission of light through the window and at the same time are such by the application of elecricity to suitable bus bars that it is possible to pass current through them and thereby heat the windshield for defogging or deicing.
Despite the manner in which the windshield or back light heating defrosting/defogging pattern is produced, it is likely that upon the manufacturing process, one or more breaks in the intended circuit pattern will occur. It is difficult to detect such breaks and is time consuming to view the piece of glass through a microscope so as to visually detect discontinuities.
One method of detecting the broken heater lines is described in U.S. Pat. No. 3,590,371 to Shaw. The Shaw patent teaches the use of detecting circuit discontinuities in glass sheets having conductor members embedded therein by placing in association with the glass a stratum of cholesteric-phase liquid-crystal material having appropriate color change temperature-range characteristics. The Shaw reference discloses passing current through the conductors and observing color changes in the vicinity of the operating conductors in the liquid crystal material. In practice, the Shaw invention includes a manual operation of placing a sheet of the temperature sensitive liquid crystal material over the inside surface of the horizontally positioned back light while power is applied to the heater grid. By observing the changing patterns in the liquid crystal material over a short period of time as the adjacent heater lines become hot by observing an ammeter in the power circuit the operator can tell which, if any, lines are broken and the amount of total current passing through the heater pattern. The operator after disconnecting the power and removing the liquid crystal sheets then records the observations manually.
Whereas the Shaw detection method produces good results in detecting discontinuities, due to the necessary manual operation it is not entirely suitable for an automatic detection system. Such an automatic detection system should be able to observe the broken or discontinuitous heater lines while the windshield or back light is passing through the manufacturing process. The detection system should not only record that a discontinuity has occurred but it should note the location and display the information so that the discontinuity can be repaired. It would be helpful if the method of detecting the discontinuity would not contact the glass, and would be capable of detecting the discontinuity as the glass part was moved underneath or would provide a movable probe over or under the heater lines so as to not produce marring or scratching. Further, it would be beneficial if such a testing inspection apparatus were provided that would be able to detect broken lines in silk screened pattern ceramic silver or silver oxide frit prior to being fused into the substrate as such a detecting means would enable the manufacturer to more conveniently repair discontinuitous lines prior to having those lines fused onto the glass substrate.
In 1879 E.H. Hall at John Hopkins University discovered that if a conductor while carring an electric field longitudinally was placed in a magnetic field with the conductor perpendicular to the direction of the field, that there was a difference of electric potential on either side of conductor. He also observed that if such points were joined through a sensitive galvanometer that a feeble current would be indicated. If such a Hall effect instrument is utilized in conjunction with a gaussmeter, the magnetic field surrounding a conducting material can be detected. The resulting device is known as a "Hall effect" probe.